calibration

Edit 0 4 …

Initial testing by adding bolts one at a time and measuring voltage with CRO - 16 Sep
calibration.xls

  • The scale is going to be nonlinear
  • The output of the device is not going to be 0-5V, more likley 1.3 - 3V, etc.

A new calibration test has been performed, this time with masses up to 1kg. It shows very linear result, which is good.
calibration data 8-10-08.xls
Whilst using the generated formula to calculate values is somewhat inaccurate (mostly +/- 2g), these voltages are only to 3 significant figures. Hopefully the increased precision that the microcontroller gets will allow it to be more precise. Maybe.

Basic method for calibration. See... um... elsewhere... for all the #defines and globals.
/**
 * Use two points, and simple maths, to work out a calibration line. We need to work out if it's computationally
 * feasible to do it with more points.
 */
void calibrate(unsigned int zeroVoltage, unsigned int secondVoltage, unsigned in secondMass, char massUnit)
{
    float gradient = secondMass / (secondVoltage - zeroVoltage);
    float yIntercept = - zeroVoltage / gradient;
 
    voltageToMassEquation[massUnit] = {gradient, yIntercept};
 
    char otherMassUnit = massUnit ^ 1;    //0 or 1
    voltageToMassEquation[otherMassUnit] = {convert_unit(gradient, massUnit, otherMassUnit), convert_unit(yIntercept, massUnit, otherMassUnit)};
}